The present invention relates to spinal fixation methods and systems. In particular, the invention concerns a fixation plate and bone screw system for attachment to vertebral bodies to hold the vertebrae in alignment.
Within the last few decades, spinal fixation systems have been developed using generally rigid plates which are engaged between vertebrae of the spine, typically on opposite sides of the spinous processes. One such type of spinal plate is illustrated in the patents to Steffee, U.S. Pat. Nos. 4,611,581 and 4,696,290. In this device, a spinal plate is provided which has a series of openings therein for receiving the threaded portions of force transmitting members. The force transmitting members comprise a dual-threaded screw which includes a first threaded portion for engaging the bone of the vertebrae, and a second threaded portion which projects outwardly from the vertebrae and through the openings in the spinal plate. The dual-threaded bone screws in the Steffee apparatus are engaged to the bone plate by way of a nut which is tightened onto the second threaded portion of the force transmitting members and against the plate. The spinal plate described in the Steffee references includes a number of bridge elements between the elongated openings in the plate which tend to increase the rigidity of the plate.
Another type of spinal fixation system is shown in the patent to Luque, U.S. Pat. No. 4,790,297, which has been assigned to the assignee of the present invention. In this system, a spinal fixation plate includes a single elongated opening which extends substantially across the length of the plate. A number of depressions are formed in the top surface of the plate at the edge of the central elongated opening. These depressions are used to engage a convex portion of a bone screw which extends through the opening in the bone plate and into the vertebral bone. The system also includes a plate ring which encircles the plate to prevent the plate from spreading when the bone screws are tightened down onto the bone plate. The plate ring can be situated at any position along the length of the fixation plate.
One difficulty encountered by prior plating fixation systems is the failure of the pedicle screw. For instance, with the Steffee system, the first threaded portion (or bone screw portion) includes a nut surface which contacts the bottom face of the fixation plate when the separate nut is tightened against the top face of the plate. In practice it has been found that the nut surface does not contact flush against the bottom face of the fixation plate, thereby leaving an unsupported section of the second threaded or nut engagement portion exposed beneath the plate. This results in stress risers in the second threaded portion which are susceptible to fatigue failure under the cyclic loads generated by spinal motion. An increase in the angular deviation or angulation of the screw to the fixation plate in the Steffee system may increase the stresses and the likelihood of fatigue failure.
Another related disadvantage of some of the prior spinal fixation systems using a generally rigid bone plate is that the plates do not permit a beneficial amount of angulation between the bone screws and the plate, which may be required to account for various angles of fixation to the vertebrae or to accommodate normal sagittal curvature of the spine. In these prior devices, this angulation is often provided by bending the bone plate itself to conform to curvature between adjacent vertebrae. Bending the plate presents difficulty in properly aligning the pedicle screw with the vertebrae.
Optimally, in plate spinal fixation systems, a compromise in stiffness and rigidity of the system is required so that the system is stiff enough to challenge the bone to facilitate bone fusion of the spine, and not so rigid that normal loads on the spine will lead to fatigue failures of the bone screws. Present fixation systems do not appear capable of achieving this compromise.
There is therefore a need for a spinal fixation system which can more easily compromise the rigidity needs for bone growth and for protection of the fixation system components. In other words, the need exists for a fixation system that has increased stiffness characteristics without the accompanying tendency for fatigue failures.
There is also a need for a spinal fixation system that is capable of achieving angulation between the bone-engaging screws without the necessity of bending and forming the bone plate itself. A further need exists for a spinal fixation system in which the load transmitted from the vertebral bone to the force transmitting member can be transmitted to the fixation plate at an angle other than generally perpendicular to the plate surface.